An image display device that outputs an image to an observer, such as a liquid crystal display, plasma display, rear-projection display, organic EL, and FED, is provided with an image source and an optical sheet comprising layers with various functions for improving the quality of the image light emitted from the image source to output the image to the observer.
Heretofore, there have been disclosed several techniques related to such an optical sheet, one of which is disclosed in Patent Document 1 for example. Patent Document 1 discloses a two-dimensional viewing angle enlarging member (being equivalent to the “optical sheet” of the present application) comprising a base film which transmits light and has flat surfaces on both sides, wherein two light-diffusing sheets having a plurality of unit lenses with a substantially trapezoidal cross section formed in a one-dimensional or two-dimensional direction (the light-diffusing sheet being equivalent to an “optical functional layer” of the present application), are adhered to both surfaces of the base film in a manner sandwiching the base film with the phases of the sheets shifted by about 90; in each of the light-diffusing sheets, the unit lens in its trapezoidal shape has a light input part on the lower base, a light output part on the upper base, and a total reflection part on the oblique line; the unit lens is formed of a high-refractive index substance; the triangular cross section part sandwiched by the adjacent unit lenses is formed of a low-refractive index substance having a refractive index lower than that of the high-refractive index substance; and light-absorbing particles are added to the low-refractive index substance. In addition, Patent Document 1 also discloses a method for manufacturing a light-diffusing sheet.